This invention relates to liquid metal cooled fast breeder nuclear reactors of the pool kind.
In a nuclear reactor of the pool kind the reactor core and ancillary components, such as heat exchangers and coolant circulating pumps, are immersed in a pool of coolant.
One known construction of liquid metal cooled fast breeder nuclear reactor of the pool kind comprises a primary vessel suspended from the cover of a concrete vault and containing a pool of liquid sodium. The reactor core, coolant pumps and heat exchangers are also suspended from the cover of the concrete vault and are immersed in the pool of sodium. Ducting is provided whereby coolant from the pool can be pumped successively through the core and the heat exchangers thence back to the pool. Replacement of spent fuel is carried out whilst the reactor is shut down and the irradiated fuel assemblies are withdrawn from the core by a charge machine located above the cover of the concrete vault. Whilst still submerged in liquid sodium the irradiated fuel is transferred sideways and lowered into a transfer rotor. Subsequently the irradiated fuel is removed from the vault to storage racks. The transfer rotor has provision for the temporary storage of twenty irradiated fuel assemblies, each within a stainless steel bucket. For insertion and withdrawal of a fuel assembly the rotor is rotated to bring an assembly to a loading or discharging station. An irradiated fuel assembly is withdrawn from the rotor within its bucket and the combination of fuel assembly, bucket and sodium (weighing approximately 100 kilograms) is lifted approximately 27 meters. Thus in the event of malfunction the combination falls back into the rotor with considerable energy. To prevent damage to the rotor an arrangement of fracture pins limits the transfer of energy to the rotor and deformable energy absorbing devices prevent damage to the reactor vessel and other parts of the reactor structure. After such a malfunction it is necessary to replace the fractured pins and the deformed energy absorbing device by remote means operated over a considerable distance from outside the vault and the operation is, therefore, very complex. It is an object of the present invention to provide a liquid metal cooled fast breeder reactor of the pool kind having improved energy absorbing means for fuel assemblies falling into a fuel transfer rotor.